1. Field
Exemplary embodiments relate to devices for molding plastic preforms into plastic containers and in particular to stretch blow molding machines.
2. Description of Related Art
Stretch blow molding machines have been available for a long time. In conventional stretch blow molding machines, a plastic preform to be molded is placed in a blow mold and is subsequently, for example, by means of pressurized air, expanded against a wall of the blow mold to form a plastic container.
The entire blow molding operation is relatively complex and involves a plurality of process steps. For example, blow mold halves are closed around the preform and also a bottom plate is usually moved up to these blow mold halves. Further, a blow nozzle is positioned at the mouth of the preform and subsequently supplies pressurized air to the preform. It is usually necessary to lock the blow mold around the plastic preform.
In the prior art, all of these mechanical movements are carried out by means of forming cams or guiding cams. These guiding cams offer the advantage that an exactly predetermined course of movement of the individually described movements may be carried out and that a reliable blow molding process is possible even at high operating speeds.
Further, it is known from the prior art to additionally stretch the preforms in their longitudinal direction by means of a stretching rod. To this end, the stretching rod is inserted into the plastic preform during the actual molding process and stretches the preform in its longitudinal direction. In this case, no accurate timing coordination with the mechanical operations mentioned above is required. Instead, however, an accurate timing coordination with the pressurized air supply into the plastic preform is required.
A servo stretching assembly for a blow molding machine is known, for example, described in U.S. Pat. No. 5,269,672, incorporated herein by reference in its entirety. In this assembly, a stretching rod is driven by a servomotor in order to achieve an accurate and repeatable stretching movement.
U.S. Pat. No. 4,128,383, incorporated herein by reference in its entirety, describes an apparatus for controlling the distribution of the plastics material during the production of plastic containers.
A method and a device for blow molding containers described in German Patent Application No. 10 2007 008 023 A1, incorporated herein by reference in its entirety discloses that the positioning of a stretching rod is determined by means of an electromechanical stretching rod drive, wherein a rotational movement of a motor shaft of a servomotor is converted into a lifting movement of the stretching rod by means of a mechanical coupling device.
A rotational stretch blow molding machine having a magnetically driven stretching aid is known, for example, described in German Patent Application No. DE 600 03 238 T2, incorporated herein by reference in its entirety. More specifically, in the case of the blow molding stations known today, as mentioned above, the individual processes are driven by means of cams, for example, a bottom mold is driven up via an outside cam. Subsequently the mold carrier halves are closed via a cam and as soon as they rest against each other, the mold carrier halves are locked by means of a locking mechanism, whereas this operation, too, is carried out by means of a cam. Finally, a blow piston is lowered onto the mold in this condition via another cam, whereby the blow mold is sealed with respect to a high pressure valve. As soon as the blow piston or the blow nozzle is positioned on the blow mold, the stretching operation is initiated via of an additional cam.
In the prior art, however, it is sometimes necessary to convert a blow molding machine to accommodate different container types. To this end, some of the movements described above may have to be altered. To implement such a change, for example, the guiding cams mentioned are exchanged or modified. This may at times be a complex and time-consuming process.